1. Field of the invention
The present invention relates to a laser interferometer for measuring length and other physical quantities. More specifically, the present invention relates to a 3-frequency heterodyne laser interferometer and a method for measuring lengths by using the interferometer, in which the secondary beat frequency shifts between three frequencies are measured by using a 3-frequency laser as the optical source for the heterodyne interferometer, thereby doubling the resolution.
2. Description of the Related Art
In the conventional 2-frequency heterodyne interferometer using the Zeeman stabilizing laser, a primary beat frequency between two modes (2 frequencies) is used for detecting the interfered signals. However, if a laser oscillator capable of oscillating in 3 longitudinal modes is formed, and if the three longitudinal modes of the laser are called f.sub.1, f.sub.2 and f.sub.3 (assuming f.sub.3 &gt;f.sub.2 &gt;f.sub.1), the secondary beat frequency is defined to be .vertline.f.sub.3 +f.sub.1 -2f.sub.2 .vertline.. In view of the fact that the primary beat frequency value is defined to be .vertline.f.sub.1 -f.sub.2 .vertline. in the case of the two modes (2 frequencies), a factor 2 is added to the value of f.sub.2.
The inventor found that, if a 3-frequency heterodyne interferometer is used, the resolving power is doubled in measuring physical quantifies, thereby forming a 3-frequency heterodyne laser interferometer.
Prior to the present invention, the inventor disclosed a method for stabilizing the frequency and the output of a 3-frequency laser in Korean Patent Application No. 20544/1991.
According to the present invention, the resolving power is not only doubled compared with the conventional 2-frequency heterodyne laser interferometer, but also an RF electronic device such as an RF mixer for detecting the phase shift of signals of several hundred MHz (which is the difference between the two modes) is not required. Further, the laser output becomes larger, and therefore, a multi-axis laser interferometer can be manufactured. Further, there is no lowering of the accuracy, which is caused by the feedback of the beams into the laser by being reflected from the laser interferometer.